The Australian Transport Safety Bureau (ATSB) is Australia's national transport safety investigator. The ATSB's function is to improve safety and public confidence in the aviation, marine and rail modes of transport. The ATSB is Australia's prime agency for the independent investigation of civil aviation, rail and maritime accidents, incidents and safety deficiencies.

Analysis

Summary

Blade Failure

Fatigue cracking initiates and propagates in response to the
application of repeated tensile stresses. In general terms, the
greater the magnitude of the stresses, the fewer cycles are
required to produce cracking. It is possible to mathematically
model the distribution of service stresses within a component to
gather some appreciation of any areas of stress concentration that
may predispose the item to fatigue cracking.

In this case, the fatigue crack origins correlated closely with
the regions of highest stress concentration as predicted by the
mathematical analysis. This directly implies that the cracking was
a response to blade stresses induced in service, and not a product
of an isolated defect or anomaly within the component. Indeed, no
such deficiencies were found within the failed component.

Service stresses within the blade section vary in response to
the thrust rating and operation of the engine, with higher ratings
and power settings equating to higher component stresses.
Operations of high thrust rated variants of the Trent 800 series
engine, fitted to high gross weight aircraft, would thus be
expected to produce the highest level of service stress related
failures. The Boeing 777-300 subject aircraft was a typical example
of this situation.

The uniform transmission of service loads through a multiple
component assembly requires that the contact surfaces of each item
are closely matched and that these surfaces behave in a similar way
when loaded. Irregularities within these surfaces can interfere
with the even transmission of load, leading to the development of
areas of elevated stress. The surface galling and welding damage
found across the blade dovetail faces is such an irregularity and
would increasingly have interfered with the desired uniform bedding
across the tapered blade seat. Titanium and related alloys have a
high potential for surface galling and require efficient and
effective surface lubrication to prevent damage. Dry film
lubricants are often used in these applications and, while
effective, they must be maintained within the interface in order to
remain so. The surface movement that produces the galling damage
also tends to progressively force out any lubricant from the
interface, necessitating periodic re-lubrication of the assembly if
the joint is to be maintained in a low friction condition.

A modification of the dry film lubricant system was introduced
on a non-mandatory level by service bulletin RB.211-72-C905 (June
2000). This recognised the deterioration of the lubricant film and
incorporated a base layer of plasma spray coated anti-galling
compound above which a reduced thickness of the dry lubricant is
applied. The base coating acts to improve the bonding and retention
of the dry lubricant. The blades of the subject engine did not have
this modification.

Cabin safety

The premature release of seat belts by some passengers increased
the risk of injury, even though no injuries were reported.

The landscape camera system may be of value as a tool to gather
information in an emergency situation. However, camera images have
the potential to disturb or distract passengers at a time when
flight or cabin crew members may need to issue emergency
instructions or pass on important information.